Passive optical networks with mode coupling receivers

ABSTRACT

An apparatus comprising a mode coupler configured to couple a plurality optical signals into a plurality of modes, and a receiver coupled to the mode coupler and configured to detect the modes to obtain the optical signals, wherein the optical signals are coupled from single mode fibers. Also disclosed is an apparatus comprising a plurality of single mode waveguides configured to transport a plurality of single mode signals, and a detector coupled to the single mode waveguides and configured to detect the single mode signals, wherein the single mode signals are substantially coupled without loss from the single mode waveguides to the detector. Also disclosed is a method comprising receiving a plurality of single mode optical channels, coupling the single mode optical channels into a multimode channel, and detecting the optical modes corresponding to the channels in the multimode channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication 61/110,384, filed Oct. 31, 2008 by Ning Cheng et al., andentitled “Passive Optical Networks with Mode Coupling Receivers,” whichis incorporated herein by reference as if reproduced in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

The increase in Internet traffic and emerging multimedia applications,such as video on demand, high definition Television (TV), videoconference, and interactive online games requires an increase inbandwidth of access networks. To satisfy the increase in bandwidth andsupport such applications, Time Division Multiplexing (TDM) PassiveOptical Networks (PONs), such as Gigabit PONs (GPONs) and Ethernet PONs(EPONs), are currently deployed worldwide to potentially serve millionsof users. Traditionally, the maximum transmission distance of a PON isless than or about 20 Kilometers (km) and the splitting ratio is fromabout 1:16 to about 1:64, as defined in International TelecommunicationUnion (ITU) Telecommunication Standardization Sector (ITU-T) andInstitute of Electrical and Electronics Engineers (IEEE) standards. Thesplitting ratio is the ratio of one central office equipment, e.g. anOptical Line Terminal (OLT), to a plurality of user equipments, e.g.Optical Network Terminals (ONTs).

Recently, there has been interest in long-reach and large splittingratio PONs that have transmission distances larger than about 20 km andsplitting ratios larger than about 1:64. In such long-reach and largesplitting ratio PONs, the number of central offices, which can serve thesame quantity of user terminals, can be substantially reduced.Additionally, the hierarchy of the PON can be simplified, the equipmentand operation cost can be reduced, and the quality of service forreal-time traffic (e.g. video on demand) can be improved due to thereduced number of hops in the system. Accordingly, ITU-T has defined astandard (ITU-T G.984.6) for GPONs with reach extension. In thisstandard, optical amplification and/or Optical-Electrical-Optical (OEO)regeneration are considered for long-reach PON implementations. Usingoptical amplifiers or optical regenerators, long-reach PONs havingtransmission distances up to about 100 km have been demonstrated.However, the optical amplifiers or generators are active equipment thatcan increase the cost and/or maintenance requirements in the system,which may be unattractive for large scale deployment. Achievinglong-reach PONs without optical amplifiers or regenerators remainsattractive but difficult.

SUMMARY

In one embodiment, the disclosure includes an apparatus comprising amode coupler configured to couple a plurality optical signals into aplurality of modes, and a receiver coupled to the mode coupler andconfigured to detect the modes to obtain the optical signals, whereinthe optical signals are coupled from single mode fibers.

In another embodiment, the disclosure includes an apparatus comprising aplurality of single mode waveguides configured to transport a pluralityof single mode signals, and a detector coupled to the single modewaveguides and configured to detect the single mode signals, wherein thesingle mode signals are substantially coupled without loss from thesingle mode waveguides to the detector.

In yet another embodiment, the disclosure includes a method comprisingreceiving a plurality of single mode optical channels, coupling thesingle mode optical channels into a multimode channel, and detecting theoptical modes corresponding to the channels in the multimode channel.

These and other features will be more clearly understood from thefollowing detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is a schematic diagram of an embodiment of a PON.

FIG. 2 is a schematic diagram of an embodiment of a long-reach/largesplitting PON.

FIG. 3 is a schematic diagram of an embodiment of a fiber coupler.

FIG. 4 is a schematic diagram of an embodiment of a waveguide coupler.

FIG. 5 is a schematic diagram of an embodiment of a fused fiber coupler.

FIG. 6 is a schematic diagram of an embodiment of an improved fusedfiber coupler.

FIG. 7 is a schematic diagram of an embodiment of a fused fiber-lenscoupler.

FIG. 8 is a schematic diagram of an embodiment of an improved fusedfiber-lens coupler.

FIG. 9 is a schematic diagram of an embodiment of a prism coupler.

FIG. 10 is a schematic diagram of an embodiment of a waveguidephotodiode coupler.

FIG. 11 is a flowchart of an embodiment of a long-reach/large-splittingPON detection method.

DETAILED DESCRIPTION

It should be understood at the outset that although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

Disclosed herein are systems and apparatus for deployinglong-reach/large splitting PONs without using optical amplifiers andregenerators. The long-reach/large splitting PON systems comprise alow-loss mode coupling receiver, for instance at the OLT, which may beconfigured to increase the maximum transmission distance and/or thesplitting ratio of the PONs. The PON systems may comprise a plurality ofdifferent low-loss mode coupling receivers, which may be based ondifferent schemes for coupling a plurality of single mode opticalchannels into a multimode optical channel. The different low-loss modecoupling receivers may include a fiber coupler, a waveguide coupler, afused fiber coupler, an improved fused fiber coupler, a fused fiber-lenscoupler, an improved fused fiber-lens coupler, a prism coupler, and awaveguide photodiode coupler. The low-loss mode coupling receivers mayimprove the power budget in long-reach and long-splitting PONdeployment.

FIG. 1 illustrates one embodiment of a PON 100. The PON 100 comprises anOLT 110, a plurality of ONTs 120, and an ODN 130, which may be coupledto the OLT 110 and the ONTs 120. The PON 100 may be a communicationsnetwork that does not require any active components to distribute databetween the OLT 110 and the ONTs 120. Instead, the PON 100 may use thepassive optical components in the ODN 130 to distribute data between theOLT 110 and the ONTs 120. In an embodiment, the PON 100 may be a NextGeneration Access (NGA) system, such as a ten gigabit per second (Gbps)GPON (XGPON), which may have a downstream bandwidth of about ten Gbpsand an upstream bandwidth of at least about 2.5 Gbps. Alternatively, thePON 100 may be any Ethernet based network, such as an EPON defined bythe IEEE 802.3ah standard, a 10 Gigabit EPON as defined by the IEEE802.3av standard, an asynchronous transfer mode PON (APON), a broadbandPON (BPON) defined by the ITU-T G.983 standard, a GPON defined by theITU-T G.984 standard, or a wavelength division multiplexed (WDM) PON(WPON), all of which are incorporated herein by reference as ifreproduced in their entirety.

In an embodiment, the OLT 110 may be any device that is configured tocommunicate with the ONTs 120 and another network (not shown).Specifically, the OLT 110 may act as an intermediary between the othernetwork and the ONTs 120. For instance, the OLT 110 may forward datareceived from the network to the ONTs 120, and forward data receivedfrom the ONTs 120 onto the other network. Although the specificconfiguration of the OLT 110 may vary depending on the type of PON 100,in an embodiment, the OLT 110 may comprise a transmitter and a receiver.When the other network is using a network protocol, such as Ethernet orSynchronous Optical Networking/Synchronous Digital Hierarchy(SONET/SDH), that is different from the PON protocol used in the PON100, the OLT 110 may comprise a converter that converts the networkprotocol into the PON protocol. The OLT 110 converter may also convertthe PON protocol into the network protocol. The OLT 110 may be typicallylocated at a central location, such as a central office, but may belocated at other locations as well.

In an embodiment, the ONTs 120 may be any devices that are configured tocommunicate with the OLT 110 and a customer or user (not shown).Specifically, the ONTs 120 may act as an intermediary between the OLT110 and the customer. For instance, the ONTs 120 may forward datareceived from the OLT 110 to the customer, and forward data receivedfrom the customer onto the OLT 110. Although the specific configurationof the ONTs 120 may vary depending on the type of PON 100, in anembodiment, the ONTs 120 may comprise an optical transmitter configuredto send optical signals to the OLT 110 and an optical receiverconfigured to receive optical signals from the OLT 110. Additionally,the ONTs 120 may comprise a converter that converts the optical signalinto electrical signals for the customer, such as signals in theEthernet or asynchronous transfer mode (ATM) protocol, and a secondtransmitter and/or receiver that may send and/or receive the electricalsignals to a customer device. In some embodiments, ONTs 120 and opticalnetwork units (ONUs) are similar, and thus the terms are usedinterchangeably herein. The ONTs 120 may be typically located atdistributed locations, such as the customer premises, but may be locatedat other locations as well.

In an embodiment, the ODN 130 may be a data distribution system, whichmay comprise optical fiber cables, couplers, splitters, distributors,and/or other equipment. In an embodiment, the optical fiber cables,couplers, splitters, distributors, and/or other equipment may be passiveoptical components. Specifically, the optical fiber cables, couplers,splitters, distributors, and/or other equipment may be components thatdo not require any power to distribute data signals between the OLT 110and the ONTs 120. Alternatively, the ODN 130 may comprise one or aplurality of active components, such as optical amplifiers. The ODN 130may typically extend from the OLT 110 to the ONTs 120 in a branchingconfiguration as shown in FIG. 1, but may be alternatively configured inany other point-to-multi-point configuration.

The PON 100 may have a maximum transmission distance less than or equalto about 20 km and a splitting ratio less than or equal to about 1:64.For instance, a plurality of splitters may be used in the ODN 130 tosplit each branch of fiber into a plurality of branches until reachingsuch splitting ratio. Typically, to increase the splitting ratio andincrease the maximum transmission distance of the PON 100, a pluralityof optical amplifiers and/or regenerators may be added, for instance tocouple some of the fiber cables in the ODN 130 and thus boost theoptical signal power for longer reach and/or larger splitting ratio.However, such combination of splitters and optical amplifiers (orregenerators) may increase the cost of deployment, which may not bedesirable or practical.

FIG. 2 illustrates one embodiment of a long-reach/large-splitting PON200, which may have an increased maximum transmission distance, e.g.greater than about 20 km. Additionally or alternatively, thelong-reach/large-splitting PON 200 may have an increased splittingratio, e.g. greater than about 1:64. Specifically, thelong-reach/large-splitting PON 200 may be configured for longer maximumtransmission distance and/or larger splitting ratio without using anoptical amplifier or regenerator in the system. Thelong-reach/large-splitting PON 200 may comprise an OLT 210, a pluralityof ONTs 220, and an ODN 230 coupled to the OLT 210 and the ONTs 220.Similar to the PON 100, the long-reach/large-splitting PON 200 may be aGPON, EPON, APON, BPON, WPON, or NGA system.

Similar to the OLT 110, the OLT 210 may be configured to communicatewith the ONTs 220 and another network (not shown) and may act as anintermediary between the other network and the ONTs 220. The OLT 210 maycomprise a receiver (Rx) 211, a transmitter (Tx) 212, and a plurality ofoptical signal separators 213. The optical signal separators may be anydevices configured to separate upstream and downstream optical signalsat the OLT 210. For instance, the optical signal separators 213 may beWDM diplexers or optical circulators that receive the upstream opticalsignals from the ODN 230 via a plurality of first fibers and forward theupstream optical signals to the RX 211 via a plurality of second fibers.The first fibers may be configured for bidirectional transmission fromand to the ONTs 220 and the second fibers may be single mode fibers. Theoptical separators 213 may also receive the downstream optical signalsfrom the Tx 212 via a third plurality of fibers and forward thedownstream optical signals to the ODN 230 via the first fibers.

Additionally, the OLT 210 may comprise a mode coupler 214 coupled to theRx 211 and the optical signal separators 213, and an OLT splitter 215coupled to the Tx 212 and the optical signal separators 213. The modecoupler 214 may be any device configured to couple the upstream opticalsignals from the ONTs 220 into the Rx 211. The upstream optical signalsmay be forwarded to the Rx 211 via the ODN 230, which may be coupled tothe optical signals separators 213. Specifically, the mode coupler 214may couple the upstream optical signals into different optical modes,for instance similar to a space division multiplexing scheme. Thecoupled upstream optical signals may be forwarded to the Rx 211 andhence detected. The Rx 211 may detect a plurality of coupled modes orchannels corresponding to the upstream optical signals. Coupling theupstream optical signals into different optical modes may reduce theinsertion loss for each optical signal and thus improve detection, incomparison to conventional receiver schemes that use optical splitters.Reducing the insertion loss in the optical detection scheme may increasethe power budget of the system for upstream transmission, which mayextend the maximum transmission distance for the upstream opticalsignals from the ONTs 220 to the OLT 210. Additionally, increasing thepower budget may allow a larger splitting ratio for serving more ONTs220 in the system. As such, using the mode coupler 214 may improve thelong-reach and splitting ratio capabilities of the system without addingoptical amplifiers or regenerators.

The OLT splitter 215 may be any device configured to split thedownstream optical signals from the Tx 212 into a plurality ofdownstream signal copies, which may be forwarded to the optical signalsseparators 213. The optical signal separators 213 may forward thedownstream optical signals to the ODN 230. In comparison to the increasein power budget provided by the mode coupler 214, the OLT splitter 215may provide no or less substantial increase in the power budget fordownstream transmission. However, the downstream optical signals may betransmitted at a wavelength equal to about 1490 nanometers (nm), whichmay suffer lower fiber losses than the upstream optical signals (e.g. atabout 1310 nm). Therefore, the long-reach and splitting ratiorequirements of the system for downstream optical signals may be lowerthan the requirements for upstream optical signals. Hence, using thecombination of the mode coupler 214 and the OLT splitter 215 may providean improved overall long-reach and splitting ratio transmission in thesystem.

The ODN 230 may comprise a plurality of ODN splitters 232 that receivethe downstream optical signals form the OLT 210. The ODN splitters 232may be any devices configured to split the downstream optical signalsfrom the OLT 210 into a plurality of downstream signal copies. Thedownstream signal copies may be forwarded to the ONTs 220, which may beconfigured similar to the ONTs 130. Specifically, each of the ODNsplitters 232 may be coupled to the optical signal separators 213 viathe first fiber cables, e.g. bidirectional fiber cables, and to aplurality of corresponding ONTs 220 via another plurality of fibercables similar to the first fiber cables. In an embodiment, the ONTs 220may be coupled to each ODN splitter 232 via a plurality of fiber cables,which may be coupled in parallel in a single aggregate cable. In analternative embodiment the ODN splitters 232 may be positioned at thecentral office with the OLT 210 instead of the ODN 230.

In an embodiment, the OLT 210 may comprise about four optical signalseparators 213, which may be each coupled to the mode coupler 214 andthe OLT splitter 215, as shown in FIG. 2. Accordingly, the ODN 230 maycomprise about four ODN splitters 232, which may be each coupled to oneof the optical signal separators 213 via a different fiber cable. EachODN splitter 232 may also be coupled to up to about eight ONTs 220 (e.g.ONT1 to ONT8). As such, the long-reach/large-splitting PON 200 may havea splitting ratio of about 1:32. In other embodiments, thelong-reach/large-splitting PON 200 may have a larger splitting ratio,such as greater than or equal to about 1:64. For instance, each of aboutfour ODN splitters 232 may be coupled to at least about 16 ONTs 220 viaseparate fiber cables.

Further, the architecture of the long-reach/large-splitting PON 200 maybe used to allow a plurality of PONs to share a single OLT port. Forexample, the OLT 210 may be coupled to a plurality of PONs, which mayeach comprise an ODN similar to the ODN 230 and a plurality of ONTssimilar to the ONTs 220. The OLT 210 may be deployed during an initialPON roll-out phase or during an upgrade phase for evolving towards nextgeneration PONs. In the initial roll-out or upgrade phase, there may berelatively few users in each PON. Accordingly, a number of PONs mayshare a single OLT port, which may save initial deployment cost.Additional OLT ports may then be added when the number of users in eachPON increases.

Table 1 shows a plurality of PON parameters for a PON configuration thatmay be used in the long-reach/large-splitting PON 200. The PONparameters may correspond to the upstream optical signals and thedownstream optical signals. The long-reach/large-splitting PON 200 mayhave a maximum transmission distance equal to about 60 km and asplitting ratio equal to about 1:32. For instance, the splitting ratiomay be equal to about 1:4 for each of the mode coupler 214 and the OLTsplitter 215 and may be equal to about 1:8 for each of the ODN splitters232. For the upstream optical signals, the PON parameters may comprisean ONT transmitter power (e.g. for any of the ONTs 220) and an OLTreceiver sensitivity (e.g. for the Rx 211). The ONT transmitter powermay be equal to about two decibels per milliwatt (dBm) and the OLTreceiver sensitivity may be equal to about −32 dBm. The upstreamsignals' parameters may also comprise a fiber loss corresponding toabout 60 km distance at about 1310 nm wavelength (for upstreamtransmission), a first splitter loss (e.g. in the ODN splitters 232),and a mode coupler loss (e.g. assuming about one decibel (dB) insertionloss). The fiber loss may be equal to about 21 dB, the first splitterloss may be equal to about ten dB, and the mode coupler loss may beequal to about one dB. Additionally, a power budget margin for upstreamtransmission may be calculated based on at least some of the remainingparameters. The power budget margin may be equal to about two dB.

Similarly, the PON parameter values for the downstream optical signalsmay comprise an OLT transmitter power (e.g. for the Tx 212), a fiberloss corresponding to about 60 km distance at about 1490 nm wavelength(for downstream transmission), a first splitter loss (e.g. in the ODNsplitter 232 having about 1:8 splitting ratio), a second splitter loss(e.g. for the OLT splitter 215 having about 1:4 splitting ratio), and anONT receiver sensitivity (e.g. for any of the ONTs 220). For thedownstream optical signals, the OLT transmitter power may be equal toabout three dBm, the fiber loss may be equal to about 15 dB, the firstsplitter loss may be equal to about ten dB, the second splitter loss maybe equal to about seven dB, and the ONT receiver sensitivity may beequal to about −32 dBm. As such, the calculated power budget margin fordownstream transmission may be equal to about three dB, which may beslightly higher than for upstream transmission. Thus, providing alow-loss mode coupling receiver for upstream transmission andconventional detection for downstream transmission may be sufficient toimprove the overall long-reach and/or splitting ratio capabilities ofthe system.

TABLE 1 Upstream Downstream ONT transmitted power 2 dBm OLT transmittedpower 3 dBm 60 km fiber loss at 21 dB 60 km fiber loss at 15 dB 1310 nm1490 nm Splitter loss (1:8) 10 dB Splitter loss (1:8) 10 dB Mode couplerloss 1 dB Splitter loss (1:4) 7 dB OLT receiver sensitivity −32 dBm ONTreceiver sensitivity −32 dBm Power budget margin 2 dB Power budgetmargin 3 dB

In an embodiment, the long-reach/large-splitting PON 200 may beconfigured to support communications for about 64 ONTs 220, e.g. mayhave a splitting ratio equal to about 1:64. For example, the splittingratio of the mode coupler 214 may be equal to about 1:2 and thesplitting ration of the ODN splitters 232 may be equal to about 1:32.Accordingly, the mode coupler 214 may be coupled to about two ODNsplitters 232, which may be each coupled to about 32 ONTs 220.Alternatively, the splitting ratio of the mode coupler 214 may be equalto about 1:4 and the splitting ration of the ODN splitters 232 may beequal to about 1:16. In another embodiment, the mode coupler 214 mayhave a splitting ratio equal to about 1:8 and the ODN splitters 232 mayalso have a splitting ratio equal to about 1:8. In other embodiments,the long-reach/large-splitting PON 200 may have a splitting ratiogreater than about 1:64, e.g. may support more than about 64 ONTs 220.For example, the splitting ratio of the mode coupler 214 may be equal toabout 1:3 or about 1:4 and the splitting ratio of the ODN splitters maybe equal to about 1:32. Alternatively, the mode coupler 214 may have asplitting ratio equal to about 1:8 and the ODN splitters 232 may have asplitting ratio equal to about 1:16 or about 1:32. Other combinations ofmode coupler 214 and ODN splitters 232 may be used to provide a combinedsplitting ration greater than about 1:64.

FIG. 3 illustrates an embodiment of a fiber coupler 300, which may be alow-loss mode coupling receiver used for mode coupling and detection ina long-reach PON. For instance, the fiber coupler 300 may comprise atleast some of the mode coupling and detection components of thelong-reach/large-splitting PON 200, e.g. a plurality of components ofthe mode coupler 214 and the Rx 211. Accordingly, the fiber coupler 300may receive a plurality of upstream optical signals from the ONTs,couple the signals into different optical modes, and detect the signalin all the modes. The fiber coupler 300 may have lower insertion lossfor each optical signal than conventional detection schemes andtherefore may increase the power budget of the system for upstreamtransmission. The fiber coupler 300 may comprise a plurality of firstfibers 310, a second fiber 320, and a detector 330.

The first fibers 310 may be single mode fibers and may transport anupstream optical signal from one of the ONTs. Alternatively, each of thefirst fibers 310 may transport a plurality of upstream optical signalsfrom a plurality of ONTs, e.g. from the ONTs 220 using the ODN splitters232. Each of the first fibers 310 may comprise a tapered tip at one end,which may be coupled to (e.g. positioned in close proximity to) one endof the second fiber 320. Additionally, the first fibers 310 may each betilted at a corresponding angle with respect to the orientation of thesecond fiber 320. To reduce the insertion loss of the first fibers 310,the angle of each first fiber 310 may be within the acceptance angle ofthe second fiber 320. The tapered tips, their corresponding angles, andthe distance between the edges of the first fibers 310 and the secondfiber 320 may be configured to improve optical mode coupling between thefirst fibers 310 and the second fiber 320. For instance, the dimensionsand orientation of the tapered tips and the distance between the taperedtips and the second fiber 320 may be designed based on the diameters ofthe first fibers 310 and the second fiber 320 to increase the amount ofoptical energy that can be coupled between the fibers and reduce theinsertion loss of the fibers. Further, the quantity of first fibers 310that may be coupled to the second fiber 320 may be based on thediameters of the first fibers 310 and the second fiber 320. Forinstance, about three or about four first fibers 310 may be coupled tothe second fiber 320 to provide about 1:3 or about 1:4 splitting ratio,respectively. In an embodiment, the tapered tips of the first fibers 310may have a lens shape to further improve optical coupling between thefirst fibers 310 and the second fiber 320.

The second fiber 320 may have a diameter that is larger than thecombined first fibers 310 and may have a length that is smaller than thefirst fibers 310. For instance, the diameter of the core of the secondfiber 320 may be larger than the cross section area of the combinedcores of the first fibers 310. For instance, the second fiber 320 may belong enough (e.g. about a few centimeters) to allow the propagation ofthe coupled modes from the first fibers 310 to the detector 330. Thedetector 330 may be an optical detector, also referred to as photosensoror photodetector, such as a photodiode, an avalanche photodiode (APD),or a photocell. The detector 330 may convert the optical signalscorresponding to the optical modes into electrical signals that may befurther processed for communication purposes. In some embodiments, thefiber coupler 300 may comprise at least one lens (not shown), which maybe positioned between the first fibers 310 and the second fiber 320 tofurther improve optical coupling between the fibers. Additionally, asilicon bench comprising a plurality of V-shaped groves may be used toalign the first fibers 310 and the second fiber 320.

FIG. 4 illustrates an embodiment of a waveguide coupler 400, which maybe another low-loss mode coupling receiver used for mode coupling anddetection in a long-reach PON. For instance, the waveguide coupler 400may comprise at least some of the mode coupling and detection componentsin the mode coupler 214 and the Rx 211. Similar to the fiber coupler300, the waveguide coupler 400 may receive a plurality of upstreamoptical signals from the ONTs, couple the signals into different opticalmodes, and detect the signals. The waveguide coupler 400 may comprise asubstrate 405, a plurality of first waveguide channels 410, a pluralityof corresponding grooves 415, a second waveguide channel 420 coupled tothe first waveguide channels, and an integrated detector 430. Thewaveguide coupler 400 may be obtained using standard fabricationprocesses, e.g. including deposition, exposure, development, etching,and/or bonding, and using semiconductor and dielectric materials.

The substrate 405 may be a semiconductor chip, such as a Silicon (Si)substrate used in the fabrication of integrated circuit andmicroelectronics. The substrate 405 may be tabular, rectangular, or diskshaped. The substrate 405 may provide a platform to support, hold, andcouple the remaining components of the waveguide coupler 400. The firstwaveguide channels 410, the second waveguide channel 420, and thedetector 430 may be positioned on top of the substrate 405. The grooves415 may also be etched on the top surface of the substrate 405 and maybe aligned with the first waveguide channels 410. For instance, eachgroove 415 may be etched under one of the first waveguide channels 410.The grooves 415 may also extend beyond the length of the first waveguidechannels 410 to one edge of the substrate 405. As such, the grooves 405may allow coupling between the first waveguide channels 410 and aplurality of fibers that may be positioned into the grooves 415 and thattransport upstream optical signals from the ONTs. For instance, thegrooves 410 may be V-shaped grooves that provide precise alignmentcontrol between the fibers and the first waveguide channels 410.

The first waveguide channels 410 may be single mode waveguides and thesecond waveguide channel 420 may be a multimode waveguide. The firstwaveguide channels 410 and the second waveguide channel 420 may beintegrated or fused on top of the substrate 405. The first waveguidechannels 410 may each be configured for single mode propagation and maytransport one of the upstream optical signals to the second waveguidechannel 420. The second waveguide channel 420 may have a larger widththan any of the first waveguide channels 410 and may be configured tocouple the upstream optical signals from the first waveguides channels410 into a plurality of propagation modes (e.g. distribution of theoptical field). The propagation modes may be transverse propagationmodes, e.g. transverse electric (TE) modes, transverse magnetic TMmodes, and/or transverse electromagnetic (TEM) modes. The secondwaveguide channel 420 and the detector 430 may also be integrated orfused on top of the substrate 405. The detector 430 may be a photodiodeor a waveguide photodiode configured to convert the optical signals ofthe different propagation modes in the second waveguide channel 420 intoa plurality of corresponding electric signals.

Similar to the first fibers 310, the first waveguide channels 410 mayeach be tilted at a corresponding angle with respect to the orientationof the second waveguide channel 420 to improve optical coupling andreduce insertion loss. The quantity of first waveguide channels 410 thatmay be coupled to the second waveguide channel 420 may be based on thewidths of the first waveguide channels 410 and the second waveguidechannel 420. For example, there may be about three or about four firstwaveguide channels 410 couple to the second waveguide channel 420. Theintegration or fusion of the first waveguide channels 410 and secondwaveguide channel 420 may also improve the optical coupling between thewaveguide channels and reduce the insertion loss. Therefore, thewaveguide coupler 400 may be a low loss waveguide coupler, which may beused in the long-reach PON to increase the power budget for upstreamtransmission. Additionally, integrating or fusing the second waveguidecoupler 420 and the detector 430 may further increase overall opticalcoupling and reduce insertion losses.

FIG. 5 illustrates an embodiment of a fused fiber coupler 500, which maybe another low-loss mode coupling receiver used for mode coupling anddetection in a long-reach PON. The fused fiber coupler 500 may compriseat least some of the mode coupling and detection components of thelong-reach/large-splitting PON 200 and may be used to couple a pluralityof upstream optical signals from the ONTs into different optical modesthat are detected. The fused fiber coupler 500 may increase the powerbudget for upstream transmission. The fused fiber coupler 500 maycomprise a plurality of first fibers 510, a fused portion 512, a secondfiber 520 comprising a core 522, and a detector 530.

The first fibers 510, second fiber 520, and the detector 530 may beconfigured similar to the corresponding components of the fiber coupler300. However, the first fibers 510 may be fused at the fused portion512, which may be coupled to the second fiber 520. Fusing the firstfibers 510 may remove an air void between the first fibers 510, wherethe fused portion 512 may have a cone shape that reduces the combineddiameter of the first fibers 510. As such, the fused portion 512 mayhave a diameter smaller than the second fiber 520, for instance whichmay be equal to about the core 522 of the second fiber 520. The core 522may be configured to confine and support the propagation of the modes,e.g. based on internal reflection effect, in the second fiber 520. Thefused portion 512 may have a plurality of cores corresponding to thefirst fibers 510, which may be coupled effectively to the single core ofthe second fiber 520.

In some embodiments, at least one lens may be positioned between themulti-core fused portion 512 and the multimode second fiber 520 tofurther improve optical coupling and reduce insertion loss. The cores inthe fused portion 512 may be closer to each other and have smallerdiameters than in the separate cores of the first fibers 510, andtherefore may have more inter-coupling of optical power between another.This inter-coupling between the cores may be tolerated as long as theoptical power from all the cores may be substantially coupled into thesecond fiber 520.

FIG. 6 illustrates an embodiment of an improved fused fiber coupler 600,which may be another low-loss mode coupling receiver used for modecoupling and detection in a long-reach PON. Similar to the fused fibercoupler 500, the improved fused fiber coupler 600 may comprise at leastsome of the mode coupling and detection components of thelong-reach/large-splitting PON 200 and may couple a plurality ofupstream optical signals from the ONTs into different optical modes. Theimproved fused fiber coupler 600 may comprise a fused portion 612comprising a plurality of first cores 614, a second fiber 620 comprisinga second core 622, and a detector 630. The fused portion 612, the secondfiber 620, and the detector 630 may be configured similar to thecorresponding components of the fused fiber coupler 500.

For instance, a plurality of first fibers (not shown), e.g. similar tothe first fibers 510, may be fused at the fused portion 612. However,each of the first cores 614 in the fused portion 612, which correspondto the cores of the individual first fibers, may be obtained by removingsome of the portion around the core (e.g. cladding) of each first fiber.The cores and remaining portions of the first fibers may be bundled orfused together to obtain the fused portion 612 and the first cores 614.As such, the first cores 614 may be closer to each other than the coresin the fused portion 512 of the fused fiber coupler 500, therebyreducing the overall diameter of the fused portion 612. The combinedcross section area of the first cores 614 may also be within thenumerical aperture of the second fiber 620, which may result in enhancedoptical coupling between the fused portion 612 and the second fiber 620.For example, the combined cross section area of the first cores 614 maybe less than or equal to about the cross section area of the second core622.

FIG. 7 illustrates an embodiment of a fused fiber-lens coupler 700,which may be another low-loss mode coupling receiver used for modecoupling and detection in the long-reach PON. For instance, the fusedfiber-lens coupler 700 may comprise at least some of the mode couplingand detection components in the mode coupler 214 and the Rx 211. Similarto the fused fiber coupler 500, the fused fiber-lens coupler 700 mayreceive a plurality of upstream optical signals from the ONTs and couplethe signals into different optical modes that may be detected. The fusedfiber-lens coupler 700 may comprise a plurality of first fibers 710, afused portion 712, and a detector 730, which may be configured similarto the corresponding components of the fused fiber coupler 500.Additionally, the fused fiber-lens 700 may comprise at least one lens740, which may be positioned between the fused portion 712 and thedetector 730 to improve optical coupling between the two components.

Unlike the mode coupling receivers above, the fused fiber-lens coupler700 may not comprise a second fiber between the first fibers 710 and thedetector 730. Instead the fused portion 712 may be coupled to thedetector 730 directly or via the lens 740. As such the optical signalsfrom the first fibers 710 may be forwarded directly to and detected bythe detector 730. Excluding the second fiber from the fused fiber-lenscoupler 700 may reduce overall losses in the fibers, e.g. bysubstantially limiting the losses to inter-coupling of optical powerbetween the cores of the first fibers 710 and the fused portion 712.

FIG. 8 illustrates an embodiment of an improved fused fiber-lens coupler800, which may be another low-loss mode coupling receiver used for modecoupling and detection in the long-reach PON. The improved fusedfiber-lens coupler 800 may comprise a fused portion 812 comprising aplurality of cores 814, a detector 830 that may be directly coupled tothe fused portion 812. Additionally, the improved fused fiber-lenscoupler 800 may comprise at least a lens 840, which may be positionedbetween the fused portion the detector 812 and 830 to improve opticalcoupling between the two components. The components of the improvedfused fiber-lens coupler 800 may be configured similar to thecorresponding components of the improved fused fiber coupler 600.However, the improved fused fiber-lens coupler 800 may not comprise asecond fiber between the fused portion 812 and the detector 830. Byeliminating the second fiber from the improved fused fiber-lens coupler800, the overall optical coupling into the detector 830 may be improvedin comparison to the improved fused fiber coupler 600. Further, sincethe cores 814 of the fused portion 812 may be closer to each other thanthe cores in the fused portion 712, enhanced optical coupling from thecores 814 may be achieved in comparison to the fused fiber-lens coupler700.

FIG. 9 illustrates an embodiment of a prism coupler 900, which may beanother low-loss mode coupling receiver used for mode coupling anddetection in the long-reach PON. The prism coupler 900 may comprise atleast some of the mode coupling and detection components in thelong-reach PON. The prism coupler 900 may comprise a plurality of firstfibers 910, a prism 918, and a detector 930. Additionally, the prismcoupler 900 may comprise at least one lens 940 between the prism 918 andthe detector 930. The prism 918 may be coupled to the first fibers 910.As such, the prism 918 may direct the optical signals towards thedetector 930. As shown in FIG. 9, the lens 940 may focus the opticalsignals onto the surface of the detector 930. The prism 918 may becoupled to about two, about three, or about four first fibers 910, whereeach first fiber 910 may be aligned with one of the surfaces of theprism 918. As shown in FIG. 9, the prism 918 may have a pyramid shapeand comprise about five faces. However, in other embodiments, the prism918 may have different shapes and may comprise any number of faces,which may be flat or curved. The remaining components of the prismcoupler 900 may be configured similar to the corresponding componentsabove.

FIG. 10 illustrates an embodiment of a waveguide photodiode coupler1000, which may be another low-loss mode coupling receiver used for modecoupling and detection in the long-reach PON. The waveguide photodiodecoupler 1000 may comprise about two first fibers 1010, a waveguidephotodiode 1030, and optionally two lenses 1040 between the first fibers1010 and the waveguide photodiode 1030. The waveguide photodiode 1030may be configured to receive the optical signals from the two firstfibers 1010 and detect the optical signals. The waveguide photodiode1030 may comprise two parallel semiconductor sections 1032, e.g. ap-type semiconductor section and an n-type semiconductor section, and anabsorptive waveguide 1034 between the two semiconductor plates 1032. Theabsorptive waveguide 1034 may absorb the optical signals from the twofirst fibers 1010 and convert the optical energy into electric current.As shown in FIG. 1000, the two first fibers 1010 may be positionedadjacent to the opposite edges of the waveguide photodiode 1030 and maybe aligned at about the height of the absorptive waveguide 1034 of thewaveguide photodiode 1030 to achieve optical coupling. The remainingcomponents of the waveguide photodiode coupler 1000 may be configuredsimilar to the corresponding components above.

FIG. 11 illustrates one embodiment of a long-reach/large-splitting PONdetection method 1100. The long-reach/large-splitting PON detectionmethod 1100 may be used in long-reach PON systems to receivecommunications over extended distances and/or to service an extendednumber of ONTs in comparison to conventional PON systems. Specifically,the long-reach/large-splitting PON detection method 1100 may be used toimprove the power budget for upstream signals from the ONTs to the OLT.For example, the long-reach/large-splitting PON detection method 1100may be used to establish communications between an OLT and a pluralityof ONTs, where the distance between the OLT and the ONTs is greater thanor equal to about 20 km. Further, the quantity of ONTs in the system maybe greater than or equal to about 32 ONTs. The method 1100 may begin atblock 1110, where a plurality of single mode optical channels may bereceived, e.g. from a plurality of different ONTs. For instance, aplurality of optical signals may be each received at the OLT viaseparate single mode fibers. Alternatively, at least some of the opticalsignals may be received via a single fiber.

At block 1120, the signals from the single mode optical channels may becoupled into a multimode channel. For instance, a low-loss mode couplingreceiver at the OLT, such as any of the mode coupling receivers, may beused to couple the optical signals from the ONTs into a plurality ofcorresponding modes in a single multimode fiber or waveguide. Combiningthe optical signals into a single multimode fiber or waveguide mayreduce the overall optical losses in the fibers, and in particular theinsertion loss of each optical signal. Reducing the optical losses inthe optical signals may provide additional power budget for transmittingthe optical signals from the ONTs. The increase in power budget may beused to extend the reach and/or splitting ratio of the optical signalsfrom the ONTs. At block 1130, the signal in the optical modes of themultimode channel may be detected. For instance, a detector may be usedto retrieve the data corresponding to the optical channels from thedifferent ONTs. The method 1100 may then end.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations should be understood to include iterative rangesor limitations of like magnitude falling within the expressly statedranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4,etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, R_(l), and an upperlimit, R_(u), is disclosed, any number falling within the range isspecifically disclosed. In particular, the following numbers within therange are specifically disclosed: R=R_(l)+k*(R_(u)−R_(l)), wherein k isa variable ranging from 1 percent to 100 percent with a 1 percentincrement, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5percent, . . . , 50 percent, 51 percent, 52 percent, . . . , 95 percent,96 percent, 97 percent, 98 percent, 99 percent, or 100 percent.Moreover, any numerical range defined by two R numbers as defined in theabove is also specifically disclosed. Use of the term “optionally” withrespect to any element of a claim means that the element is required, oralternatively, the element is not required, both alternatives beingwithin the scope of the claim. Use of broader terms such as comprises,includes, and having should be understood to provide support fornarrower terms such as consisting of, consisting essentially of, andcomprised substantially of. Accordingly, the scope of protection is notlimited by the description set out above but is defined by the claimsthat follow, that scope including all equivalents of the subject matterof the claims. Each and every claim is incorporated as furtherdisclosure into the specification and the claims are embodiment(s) ofthe present disclosure. The discussion of a reference in the disclosureis not an admission that it is prior art, especially any reference thathas a publication date after the priority date of this application. Thedisclosure of all patents, patent applications, and publications citedin the disclosure are hereby incorporated by reference, to the extentthat they provide exemplary, procedural, or other details supplementaryto the disclosure.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. An apparatus comprising: an optical transmitterconfigured to transmit a first optical signal to a plurality of opticalnetwork terminals (ONTs); a first splitter coupled to the opticaltransmitter and configured to split the first optical signal into aplurality of first signal copies; a plurality of second splitterscoupled to the first splitter, wherein each second splitter isconfigured to be positioned between the first splitter and the ONTs, andwherein each second splitter is configured to receive one of the firstsignal copies from the first splitter, forward the first signal copiesto the ONTs in a downstream direction, and forward a plurality of secondoptical signals received from at least some of the ONTs in an upstreamdirection; a plurality of separators positioned between the firstsplitter and the second splitters and configured to forward the firstsignal copies to the second splitters and forward the second opticalsignals along a plurality of single mode waveguides; a mode couplercoupled to the single mode waveguides and configured to receive thesecond optical signals and combine the second optical signals into amulti-mode waveguide; and an optical receiver coupled to the modecoupler via the multi-mode waveguide and configured to detect the secondoptical signals.
 2. The apparatus of claim 1, wherein the mode couplercomprises a prism, wherein each of the single mode waveguides arepositioned at and aligned with one of the faces of the prism, andwherein a lens is positioned between the prism and the receiver.
 3. Theapparatus of claim 1, wherein the quantity of ONTs is larger than orequal to about
 64. 4. The apparatus of claim 1, wherein the opticalsignals are transmitted at a distance larger than or equal to about 20kilometers.
 5. The apparatus of claim 1, wherein the sum of the crosssection areas of each of the single mode waveguides is less than thecross section area of the multimode waveguide.
 6. The apparatus of claim1, wherein the single mode waveguides are single mode fibers that havetapered edges coupled to the multimode waveguide, wherein the multimodewaveguide is a multimode fiber, and wherein each of the single modefibers is oriented at an angle less than or equal to about an acceptanceangle of the multimode fiber within the mode coupler.
 7. The apparatusof claim 1, wherein the mode coupler is located on only one chip, andwherein each of the single mode waveguides is oriented at an angle lessthan or equal to about an acceptance angle of the multimode waveguide onthe chip.
 8. The apparatus of claim 1, wherein the single modewaveguides are single mode fibers that have a fused portion coupled tothe multimode waveguide within the mode coupler, wherein the multimodewaveguide is a multimode fiber, and wherein the diameter of the fusedportion is less than or equal to about a diameter of a core of themultimode fiber.
 9. The apparatus of claim 1, wherein the single modewaveguides correspond to a plurality of cores in a single fiber, whereinthe multimode waveguide is a multimode fiber, and wherein the diameterof the single fiber is less than or equal to about the diameter of themultimode fiber.
 10. The apparatus of claim 1, wherein the single modewaveguides are single mode fibers that have a fused portion located inthe mode coupler, wherein the diameter of the fused portion is less thanor equal to about the cross section area of the optical receiver, andwherein a lens is positioned between the fused portion and the opticalreceiver.
 11. The apparatus of claim 1, wherein the single modewaveguides correspond to a plurality of cores in a single fiber, andwherein a lens is positioned in the mode coupler between the singlefiber and the optical receiver.
 12. The apparatus of claim 1, whereinthe optical receiver is a waveguide photodiode, wherein the single modewaveguides are positioned on opposite edges of the waveguide photodiodeand aligned with an absorptive waveguide inside the waveguidephotodiode, and wherein the mode coupler comprises a lens is positionedbetween each of the single mode waveguides and the waveguide photodiode.13. An apparatus comprising: an optical transmitter; a splitter coupledto the optical transmitter; a plurality of optical separators eachcoupled to the splitter such that the splitter is positioned between theoptical separators and the optical transmitter; and a mode couplingreceiver (MCR) coupled to each of the optical separators, wherein theoptical separators are configured to receive upstream optical signalsfrom an optical distribution network (ODN) via a plurality of firstfibers and forward the upstream optical signals to the MCR via aplurality of second fibers, and wherein the first fibers are configuredto bidirectional transmission and the second fibers comprise single modefibers and wherein the optical separators are configured to receivedownstream optical signals from the optical transmitters via a pluralityof third fibers and to forward the downstream optical signals to the ODNvia the first fibers.
 14. The apparatus of claim 13, wherein the MCR iscoupled to the optical separators via single mode fibers, wherein theoptical transmitter is configured to transmit a first optical signal,wherein the first optical signal does not pass through the single modefibers, wherein the MCR is configured to receive a second opticalsignal, and wherein the second optical signal does not pass through thesplitter.
 15. The apparatus of claim 14, further comprising a pluralityof second splitters coupled to the optical separators and to a passiveoptical network (PON) optical distribution network (ODN) interface,wherein the optical separators are positioned between the secondsplitters and the splitter, and wherein the optical separators arepositioned between the second splitters and the MCR.
 16. The apparatusof claim 13, wherein the optical transmitter is configured to transmitoptical signals to an optical line terminal (OLT).
 17. The apparatus ofclaim 13, wherein the splitter is configured to split downstream signalsfrom the transmitter into a plurality of downstream signals and isconfigured to provide a signal loss equal to about ten decibels.
 18. Theapparatus of claim 13, wherein the MCR is configured to couple theoptical signals from the optical separators into a plurality ofcorresponding modes in a single multimode fiber or waveguide, whereincombining the optical signals into the single multimode fiber orwaveguide reduces an overall optical loss in the fibers.
 19. Theapparatus of claim 1, wherein neither the first optical signals nor thefirst signal copies pass through the mode coupler, and wherein thesecond optical signals do not pass through the first splitter.